1. Field of the Invention
The present invention relates to a power supply apparatus using chargeable batteries. Portable devices such as a notebook personal computer, etc. can usually be operated by both AC mains and a battery. When the portable devices are operated by the AC mains, an AC adapter is used. The AC adaptor is designed so as to charge the batteries while also supplying a load with power. The present invention relates to a power supply apparatus for such a device installed with a plurality of chargeable batteries.
2. Description of the Related Art
FIG. 1 shows the configuration of a conventional power supply apparatus with chargeable batteries. The operation of this conventional power supply apparatus is described in detail in the following prior application.
Laid-open Patent Publication No. 8-137814 (9-322431)
Inventor: Seiya Kitagawa
Title of the Invention: Power supply apparatus
In FIG. 1, an AC adaptor is connected to the external power source terminal, power supplied from the AC adaptor drives a load 10 through a diode 9, and also charges a battery 14 through a DC-DC converter 11 for charging. When the AC adaptor is not connected or the voltage drops to an abnormally low level, the potential of the cathode side of the diode 9 falls, this fall of potential is detected by a charge/discharge monitor circuit 16, and the PWM (Pulse With Modulation) control circuit 25 inside the DC-DC converter 11 is controlled by the amount of potential drop. Thus, a FET 21 is always kept on, and the discharge current of the battery 14 is supplied to the load 10 through the DC-DC converter 11.
The DC-DC converter 11 is mainly used to regulate the voltage between the AC adaptor and the battery 14. The DC-DC converter 11 controls the charge to the battery 14 by switching on/off the FET 21 according to the control of a voltage error amplifier, a current error amplifier and a PWM comparator inside the PWM control circuit 25. For the details of these operations, see the above-mentioned prior application.
In the conventional example shown in FIG. 1, since only one chargeable battery 14 (a single package) can be used, there was a problem that the operation hours of a device driven by the battery cannot be extended by connecting a plurality of chargeable batteries in parallel. This is because when there is an imbalance in the charge states of batteries connected in parallel, energy flows from charged batteries to less-charged batteries, and such a charging overcurrent which occurs in this situation may damage the batteries.
It is an object of the present invention to provide a power supply apparatus such that the energy of the batteries may be effectively used, even if a plurality of chargeable batteries are connected in parallel, and a charge/discharge method of the batteries.
One embodiment of the present invention comprises a charge/discharge monitor unit for judging whether or not the power supply apparatus with a plurality of chargeable batteries connected in parallel between a node of an external power source and a load, and a common ground of the external power source and the load, is in a charge state when the external power source outputs voltage sufficient to drive a load and to charge the batteries, or is in a discharge state where the external power source does not output sufficient voltage and a current is discharged from the chargeable batteries to the load; switches inserted between each of the plurality of batteries, and an ON/OFF control unit for controlling the on/off operation of the switches according to the output of the charge/discharge monitor unit and the charge/discharge state of each of the plurality of batteries.
By controlling the switches for controlling the charge or discharge current of each battery, the ON/OFF control unit prevents a current from flowing back from charged batteries to less-charged batteries, if there is an imbalance in the charge states of the batteries.
Another embodiment further comprises a voltage equivalence detector unit for detecting the equivalence of battery voltages between chargeable batteries, and a battery current direction judgement unit for judging whether a current in each battery flows in a charge direction or discharge direction.
When the charge/discharge monitor unit detects a charge state, for example, out of two batteries, one battery in which current is judged to flow in a charge direction is charged by switching on/off switches inserted in series and corresponding to the other battery in which current is judged to flow in a discharge direction, by the battery current direction judgement unit. When a voltage equivalence is detected between one battery during charging and the other battery by the voltage equivalence detector circuit, the charge of the other battery is then controlled by the ON/OFF control unit.
Another embodiment further comprises a discharge completion detector unit for detecting the discharge completion state of each chargeable battery, and a battery current direction detection unit for detecting whether a current in each battery flows in a charge direction or discharge direction.
When the charge/discharge monitor unit detects a shift in status from a charge state to a discharge state, for example, out of two batteries, one battery in which a current is judged to flow in a charge direction is charged by switching on/off switches inserted in series and corresponding to the other battery in which a current is judged to flow in a discharge direction, by the battery current direction detection unit. When the discharge time reaches a predetermined value, a control is performed to repeat the processes of the current direction, detection and after by the battery current direction detector unit.
For the charge method of the plurality of chargeable batteries of the present invention, for example, the following methods are used in a power supply apparatus with a DC-DC (direct current-direct current) converter between the node of an external power source terminal and a load, and the node in parallel with a plurality of batteries for being PWM-controlled when the batteries are charged with a current, and composing a directly-connected discharge route when a current is discharged from a battery to a load.
When the DC-DC converter is constant-voltage-controlled so that an output voltage may become constant, for example, out of two batteries, only one battery in which a current flows in a charge direction when charging is started is charged, and when the voltage of the battery during charging and the voltage of the other battery during not charging become equal, a control is performed so that the battery not charged may be charged.
When the DC-DC converter is constant-current-controlled so that an output current is constant, for example, out of two batteries, one battery with a lower voltage is first charged, and when the voltage of the battery during charging and the voltage of the other battery during not charging become equal, a control is performed so that the battery not charged may also be charged.
For the discharge method of the plurality of chargeable batteries of the present invention, when the status of the power supply apparatus shifts from a battery charging state to a battery discharging state, for example, a control is performed so that out of two batteries, one battery during charging may be first discharged, and after the apparatus detects the completion of the discharging, the other battery is discharged.
As described above, according to the present invention, in a power supply apparatus where a plurality of chargeable batteries are connected in parallel, switches are switched on/off so that the charged energy of the batteries may be effectively used.